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The Enigmatic Sixth Wave of the Electrocardiogram: the U Wave

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The Enigmatic Sixth Wave of the Electrocardiogram: the U Wave Cardiology Journal 2008, Vol. 15, No. 5, pp. 408–421 Copyright © 2008 Via Medica REVIEW ARTICLE ISSN 1897–5593 The enigmatic sixth wave of the electrocardiogram: The U wave Andrés Ricardo Pérez Riera1, Celso Ferreira1, Celso Ferreira Filho1, Marcelo Ferreira1, Adriano Meneghini1, Augusto Hiroshi Uchida2, Edgardo Schapachnik3, Sergio Dubner4, Li Zhang5 1For International VCG Investigators, ABC Faculty of Medicine (FMABC), Foundation of ABC (FUABC), Santo André, Brazil 2Heart Institute of the University of São Paulo Medical School, São Paulo, Brazil 3Dr. Cosme Argerich Hospital, Buenos Aires, Argentina 4Clínica and Maternidad Suizo Argentina, Buenos Aires, Argentina 5Main Line Health Heart Center, Jefferson Medical College, Philadelphia, PA, USA Electro-Vectorcardigraphic Section, ABC Medical School, ABC Foundation, Santo André, São Paulo, Brazil Abstract The U wave is the last, inconstant, smallest, rounded and upward deflection of the electrocar- diogram. Controversial in origin, it is sometimes seen following the T wave with the TU junction along the baseline or fused with it and before P of the following cycle on the TP segment. In this review we will study its temporal location related to monophasic action potential, cardiac cycle and heart sounds, polarity, voltage or amplitude, frequency and shape- contour. We will analyze the clinical significance of negative, alternant, prominent U wave, and the difference between T wave with two peaks (T1–T2) and true U wave. Finally we will analyze the four main hypotheses about the source of U wave: repolarization of the intraven- tricular conducting system or Purkinje fibers system, delayed repolarization of the papillary muscles, afterpotentials caused by mechanoelectrical hypothesis or mechanoelectrical feedback, and the prolonged repolarization in the cells of the mid-myocardium (“M-cells”). (Cardiol J 2008; 15: 408–421) Key words: U wave, sixth wave of electrocardiogram, source hypothesis Introduction baseline or fused with it and before P of the follo- wing cycle on the TP segment. Usually, U wave has The U wave is the last, inconstant, smallest the same polarity as the T wave. Patients with ne- (£ 1 mm or 11% of voltage of the precedent T wave gative T waves and positive U waves are called with a range of 3% to 24%), rounded and upward de- Type I discordance. Patients with positive T waves flection (except VR, under normal conditions U wave and negative U waves are named Type II discordan- is only negative in VR) of the electrocardiogram ce. From 18,750 consecutively recorded ECGs, (ECG). Controversial in origin, it is sometimes seen 53 patients had type I and 26 type II discordance. following the T wave with the TU junction along the Types I and II were called group A and B respectively. Address for correspondence: Andrés Ricardo Pérez Riera, MD, Rua Sebastião Afonso, 885-Jd. Miriam 04417-100, Sao Paulo, Brazil, tel: (55 11) 5621-2390, fax: (55 11) 5625-7278/5506-0398, e-mail: [email protected] Received: 23.08.2008 Accepted: 30.08.2008 408 www.cardiologyjournal.org Andrés Ricardo Pérez Riera et al., The enigmatic sixth wave of the electrocardiogram: The U wave Figure 1. At higher rates the TP segment and U wave disappear when the T wave merges with the following P wave. Patients with negative T and negative U waves paper recorded the shadow. As the heart contracts (concordant polarity) were named as group C. Co- and relaxes repeatedly, Einthoven could record the ronary disease was slightly more common in wave’s pattern of these impulses. The electrocar- Group A (64%) than in Group B (46%, p = 0.174; diograph machine of today looks very different but NS). Coronary artery disease in Group C was extre- works on the same principle. mely common (88%; p < 0.001). Hypertension in We will study in the U wave, the temporal loca- the two discordant groups was similar: Group A tion related to monophasic action potential (AP), car- (60%) versus Group B (58%; p = NS), Group C was diac cycle and heart sounds, polarity, voltage or ampli- significantly higher (88%, p < 0.001). Left ventri- tude, frequency, normal duration and shape contour. cular enlargement (LVE) was 49% in Group A and 58% in Group B (p = NS), but Group C was signifi- Location of U wave cantly higher at 70% (p = 0.038). The authors found U wave needs to be recognized in relationship that the significance of any U wave is not indepen- to the following biological signals. dent of their respective T wave. They propose that the U wave should not be analyzed in isolation, but Monophasic action potential rather with respect to its T wave [1]. The ratio of U wave is coincident with phase 4 of AP. This the U wave and T wave amplitude is relatively con- phase of the AP is associated with diastole of the stant in all leads. The U wave is the last integrant of chamber of the heart (Fig. 2). ventricular repolarization together with ST segment, J wave (Osborn) and T wave. In a healthy person at Cardiac cycle low heart rates the PR, ST and TP segments are at In men under normal conditions, the temporal the same level and form the isoelectric line (Fig. 1). analysis of all phases of cardiac activity shows us In 1896 Einthoven [2], using an improved elec- that the U wave is registered during the protodia- trometer and a correction formula developed inde- stolic period of the cardiac cycle (diastolic isovolu- pendently of Burch, distinguishes only five deflec- metric phase and of fast filling) (Fig. 2). tions on ECG which he names P, Q, R, S, and T. The great master identified the U wave only a few Cardiac sounds years later in 1903, when he used the string galva- The U wave is concomitant to the second (S2) or nometer [3]. He published the first organized pre- third (S3) cardiac sounds. The S2 is produced by closu- sentation of normal and abnormal electrocardio- re of the aortic and pulmonary valves (A2 and P2), at grams recorded with a string galvanometer [4, 5]. the end of ventricular systole, and at the beginning of In this manuscript, Einthoven described for the first ventricular diastole, S3 sound occurs after S2 (Fig. 3). time the U wave among other waves. The U wave was detected only in ECGs made with the string ga- ECG surface lvanometer. This labeling was used routinely after The distance from the end of T wave until the tracings were made with the galvanometer(adapted apex of U wave is between 90 to 110 ms with ranges from [6]). Willem Einthoven, developed the first of heart rates of 50 to 100 beats/min. The distance electrocardiograph machine. It was a simple string end of T wave/end of U wave is 160 to 230 ms. galvanometer — capable of measuring small chan- ges in the electrical potential as the heart contrac- U wave polarity ted and relaxed. Electrodes were attached to the limbs of the patient. As the string deflected, it ob- In the frontal plane, normal U vector is located structed a beam of light and the photographic around + 60°; thus U wave is positive in II, III and www.cardiologyjournal.org 409 Cardiology Journal 2008, Vol. 15, No. 5 Figure 2. The U wave is coincident with phase 4 of monophasic action potential. The U wave in surface electrocar- diogram is located on the TP interval. It extends from the end of the T wave up to the P wave of the following cycle. Figure 3. The U wave of electrocardiogram and the contemporary moments of mechanical cycle of the heart and its relationship with the second sound. VF, and negative in VR and isoelectric in VL. Fre- ECG abnormalities in ca 93% of cases. The main cau- quently the U wave has equal polarity to the prece- ses of negative U wave on ECG are listed below [7]: ding T, i.e. positive where T also is (Fig. 4). — coronary artery disease; In precordial leads, U vector points towards the — hypertension (near 40% of cases); left and the front. Thus, U wave is positive and bet- — valvular heart disease; ter observed in V3 (between V2 and V4) (Fig. 5). — congenital heart disease; — hyperthyroidism; Causes of inverted U wave — primary cardiomyopathy; — without heart disease (ca 7% of cases). A negative U wave is highly specific for the pre- Additionally, a negative U wave is considered sence of heart disease and is associated with other an indirect criterion of LVE. 410 www.cardiologyjournal.org Andrés Ricardo Pérez Riera et al., The enigmatic sixth wave of the electrocardiogram: The U wave dyskinetic region. Certainly, the clinical usefulness of U waves remains underutilized in the spectrum of coronary artery disease. When U-wave changes are the first and only sign of ischemia, they may contribute to a decision regarding the hospital ad- mission of a patient without typical ischemic symp- toms [10]. In patients with acute ischemia, U wave inver- X sion could be explained by [11]: — abnormal relaxation pattern during the proto- diastolic phase; “protodiastolic shortening” or aftercontraction; — asynchronous segmental early relaxa- tion,which is defined as a localized early seg- mental outward motion of the left ventricular endocardium during isovolumetric relaxation; — altered left ventricular relaxation rate. Effort-induced U-wave inversion in the precor- dial leads has long been recognized as a marker of Figure 4. Normal location of U axis in frontal plane. stenosis of the left anterior descending coronary artery, but this pattern is seldom taken into acco- unt [12]. Lone U wave inversion after exercise or exercise-induced inversion of the U-wave is highly predictive of significant coronary artery disease and, more specifically, of disease of the proximal left anterior descending coronary artery (specificity: 93%; sensitivity 23%) [13].
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